In the art of making tires, it may be desirable to employ rubber vulcanizates that demonstrate reduced hysteresis loss, i.e., less loss of mechanical energy to heat. Hysteresis loss can be attributed to polymer free ends within the cross-linked rubber network, as well as the disassociation of filler agglomerates. The degree of dispersion of filler within the vulcanizate can also be a factor because increased dispersion may provide better wear resistance.
Functionalized polymers have been employed to reduce hysteresis loss and increase bound rubber. The functional group of the functionalized polymer is believed to reduce the number of polymer free ends via interaction with filler particles. Also, this interaction may reduce filler agglomeration, which can thereby reduce hysteretic losses attributable to the disassociation of filler agglomerates (i.e., Payne effect).
Conjugated diene monomers can be anionically polymerized by using alkyllithium compounds as initiators. Selection of certain alkyllithium compounds can provide a polymer product having functionality at the head of the polymer chain. A functional group can also be attached to the tail end of an anionically-polymerized polymer by terminating a living polymer with a functionalized compound.
For example, trialkyltin chlorides, such as tributyl tin chloride, have been employed to terminate the polymerization of conjugated dienes, as well as the copolymerization of conjugated dienes and vinyl aromatic monomers, to produce polymers having a trialkyltin functionality at the tail end of the polymer. These polymers have proven to be technologically useful in the manufacture of tire treads that are characterized by improved traction, low rolling resistance, and improved wear.
Because functionalized polymers are advantageous, especially in the preparation of tire compositions, there exists a need for additional functionalized polymers. Moreover, because precipitated silica has been increasingly used as reinforcing particulate filler in tires, functionalized elastomers having affinity to silica filler are needed.